The present invention relates in general to improved methods for selectively addressing a large number of remote terminals and, more particularly, to an improved method for selectively addressing a large number of television receivers for controlling the operation thereof.
Various forms of "pay television" systems are known in the art, some of the systems finding relatively widespread current use while others offer promise for increased implementation in the future. In general, all such systems normally contemplate the transmission, either by cable or over the air, of a special television signal which may be received and appropriately processed only by certain authorized television receivers. The means by which certain television receivers are so authorized to process the specially transmitted television signal while others are not varies from system to system, a rather simple example of which may comprise a coin-operated enabling device.
Subscription-television represents a somewhat more sophisticated form of such "pay television" systems. In subscription-television systems, see, for example, U.S. Pat. No. 3,202,758, a scrambled television signal is transmitted over the air and received and processed by a subscriber's television receiver which includes a special converter circuit operable for unscrambling the received signal. The unscrambling circuit may be rented by the viewer for allowing reproduction of specially transmitted programs so long as payment therefor has been made. Other forms of "pay television" include the so-called cable television systems. In these systems, the television signals are transmitted over special frequency bands such that their reception and processing may be controlled and limited to only authorized viewers or subscribers.
In systems of the foregoing type it is useful to provide a facility whereby the authorization to receive the specially transmitted programs may be selectively controlled by the broadcaster. For example, in subscription-television systems, the situation might arise where a subscriber has not tendered his rental payments. In such a case, it would be desirable to render the subscriber's converter circuit inoperative until the rental payment is received. A similar situation would arise upon the theft of a converter circuit wherein it would be desirable to render the stolen circuit inoperative. Yet another situation in which it would be desirable to provide a facility for selectively controlling the authorization to receive and process transmitted programs arises in the case of certain special programs. Assume, for example, that a flat rental fee is charged for authorization to receive most normally broadcast programs but that an additional fee is required for certain special programs, such as a heavyweight boxing match. Until the additional fee is paid, the subscriber's converter circuit would be rendered inoperative for receiving the special program although remaining able to receive otherwise normally transmitted programming.
To facilitate the foregoing, i.e. the selective control of program processing authorization, it has been proposed to identify each subscriber by a uniquely addressable binary code stored in a register or the like associated with the subscriber's converter circuit. Each subscriber's converter circuit would thereby be uniquely addressable such that its operation could be controlled as desired. It has further been proposed that addressing of selected subscribers be accomplished by transmitting the corresponding address codes together with a particular control code, during several otherwise unused horizontal scanning lines of the broadcast signal, e.g. horizontal lines 10, 11 and 12 of the vertical interval. In such a system, each subscriber's converter circuit would include suitable comparison means for comparing its associated stored address code to each transmitted address code and, in response to detection of a condition of equality therebetween, perform the operation represented by the transmitted control code, e.g. rendering the converter inoperative.
In order to implement an addressing scheme as described above, it is anticipated that address codes consisting of 20 data bits will be required to accommodate the large number of potential system subscribers. Since each horizontal scanning line of a television signal includes about 50 microseconds of active time, a single line may be used to transmit a maximum of about 25 data bits at a data bit rate of about 500 Khz. Therefore, a 20 bit address code together with a 5 bit control code requires the use of a full horizontal scanning line for transmission. Two address codes, together with the associated control codes, could then be completely transmitted during two respective horizontal scanning lines, three address codes during three respective horizontal scanning lines, and so on. Considering the use of horizontal scanning lines 10, 11 and 12 during the vertical interval for the transmission of the data, implementation of the foregoing technique, i.e. wherein each designated horizontal scanning line carries a single subscriber address code, allows the transmission of three address codes per field of the television signal or only sixty address codes per second. Although this limitation is not especially troublesome when only a small number of subscribers are being addressed, it becomes quite constraining when a large numer of subscribers need to be addressed. For example, in the previously mentioned situation where a special program is to be transmitted for which an additional fee must be paid by the subscribers, it is not unreasonable to anticipate that 5 million or more subscribers would have to be addressed for rendering their converters inoperative. Such would necessitate the transmission of address codes during a continuous time period of about ten hours which is considered to be an unacceptably long period of time. It is accordingly a primary object of the present invention to provide an addressing system utilizing a predetermined number of horizontal scanning lines, which addressing system allows for the transmission of address codes at an increased rate as compared to that achieved by transmitting a single subscriber address code during each of the designated horizontal scanning lines.